A non-aqueous secondary battery as represented by a lithium-ion secondary battery has been pervasive as the main power supply of portable electronic devices, for example, such as cellular phones and laptop computers. The lithium-ion secondary battery has been the subject of ongoing development to obtain higher energy density, higher capacity, and higher output. This trend is expected to increase in the future. To meet such demands, it is of great importance to provide a technique that ensures high battery safety.
The separator for lithium-ion secondary batteries generally uses a microporous membrane made from polyethylene or polypropylene. The separator has a function known as a shutdown function, intended to provide safety for the lithium-ion secondary battery. The shutdown function refers to the separator's ability to abruptly increase resistance when the battery temperature rises to a certain temperature. With the shutdown function, the separator shuts down the current flow when there is unexpected heat generation in the battery, preventing further temperature increase in the battery, and thereby avoiding fuming, fire, or explosion. The operating principle of the shutdown function is the closure of the pores in the separator, which occurs as the material of the separator melts and deforms. In the case of a separator made from polyethylene, the shutdown function comes into operation at a temperature of approximately 140° C., near the melting point of polyethylene. The shutdown temperature is approximately 165° C. for polypropylene separators. Because a relatively low shutdown temperature is preferred from the standpoint of ensuring battery safety, polyethylene is more commonly used for the separator.
In addition to the shutdown function, a sufficient heat resistance is required for the separator of the lithium-ion secondary battery. This is for the following reason. In conventional separators solely made from polyethylene or other microporous membranes, the separator continues to melt (known as “meltdown”) as the battery remains exposed to the operating temperatures of the shutdown function after the shutdown. This is the intrinsic characteristic of the shutdown function which operates according to the foregoing principle. The meltdown creates a short circuit inside the battery, and generates a large amount of heat, exposing the battery to the risk of fuming, fire, and explosion. The separator therefore requires, in addition to the shutdown function, a heat resistance sufficient to prevent meltdown near the operating temperatures of the shutdown function.
In an attempt to provide both the shutdown function and the heat resistance for the separator, there have been proposed separators that include a polyethylene microporous membrane coated with a porous layer made from a heat-resistant resin such as polyimide or aromatic polyamide (see, for example, Patent Documents 1 to 5). In these separators, the shutdown function comes into operation near the melting point of polyethylene (about 140° C.), and, because the heat-resistant porous layer has sufficient heat resistance, meltdown does not occur even at temperatures of 200° C. and higher. However, in this type of conventional separators, because the thickness of the polyethylene microporous membrane is as thick as about 20 μm in virtually all separators, the separator thickness exceeds 20 μm when coated with the heat-resistant porous layer. A drawback of the separators of the type provided with the heat-resistant porous layer, then, is the thickness that exceeds the thickness of about 20 μm commonly adopted by the separators currently available in the market (those solely made from polyethylene or other microporous membranes).
The shutdown function limits the thickness of the separators of the type including the heat-resistant porous layer. Specifically, because of the correlation between the shutdown function and the thickness of the polyethylene microporous membrane, the shutdown function becomes reduced when the thickness of the polyethylene microporous membrane is reduced. Further, the shutdown function tends to be reduced when the polyethylene microporous membrane is coated with the heat-resistant porous layer, compared with using the polyethylene microporous membrane alone. For these reasons, it has been required conventionally to provide a thickness of at least 20 μm for the polyethylene microporous membrane, in order to provide a sufficient shutdown function for the separator. Patent Document 3 describes as an example a polyethylene microporous membrane having a thickness of 4 μm. However, the publication does not disclose anything about the shutdown function. Usually, a sufficient shutdown function cannot be obtained when the thickness of the polyethylene microporous membrane is as small as 4 μm as in this example.
One way to reduce the separator thickness is to reduce the thickness of the heat-resistant porous layer. However, when the thickness of the heat-resistant porous layer is reduced too much, the heat resistance will be insufficient, and heat shrinkage occurs over the entire separator in a temperature range including and above the melting point of polyethylene. In this connection, Patent Document 4 teaches a configuration in which a porous layer that contains a heat resistant nitrogen-containing aromatic polymer and a ceramic powder are formed on a polyethylene microporous membrane to improve the heat resistance of the heat-resistant porous layer. This technique appears to successfully reduce the thickness of the heat-resistant porous layer without failing to provide a sufficient heat resistance. However, Patent Document 4 does not address the heat shrinkage issue of the separator, and as such a battery using the separator of Patent Document 4 has a possibility of heat shrinkage under high temperature.
As described above, concerning the separator including the polyethylene microporous membrane and the heat-resistant porous layer, no technique is available that can sufficiently cope with both the shutdown function and heat resistance issues, and, at the same time, provide a way to reduce the thickness of the separator.
From the standpoint of manufacture efficiency, there is also a need for a technique to improve the slidability of the separator provided with the heat-resistant porous layer. Specifically, battery manufacture employs a step in which a core is drawn out of the electronic element produced by winding the separator and electrodes around the core. Generally, stainless steel or other metallic material, with or without a thin ceramic coating, is used as the core. In winding, the separator is first wound around the core before the electrodes. The heat-resistant resin such as a wholly aromatic polyamide used for the heat-resistant porous layer of the separator is very adherent to the metallic material or ceramic material. The adhesion between the core and the separator causes a problem when drawing out the core from the electronic element, as it may damage the electronic element being produced.
Another challenge, then, is to improve the slidability of the separator provided with the heat-resistant porous layer. This issue becomes even more problematic in separators including the heat-resistant porous layer on the both surfaces of the polyethylene microporous membrane, because, in such separators, the heat-resistant porous layer will always be in contact with the core. The problem remains also in separators including the heat-resistant porous layer only on one surface of the polyethylene microporous membrane, because it undesirably imposes limitations on the orientation of the element, requiring the core to contact the side not provided with the heat-resistant porous layer.
The slidability issue is not addressed in the Prior Art section of Patent Document 4 or other publications. Patent Document 5 does address the slidability issue. To overcome the slidability problem, Patent Document 5 proposes a technique to form an additional spacer layer on a wholly aromatic polyamide porous layer. However, the configuration forming the spacer layer is not desirable because it involves large numbers of steps and is complex.    Patent Document 1: JP-A-2002-355938    Patent Document 2: JP-A-2005-209570    Patent Document 3: JP-A-2005-285385    Patent Document 4: JP-A-2000-030686    Patent Document 5: JP-A-2002-151044